1. Field of the Invention
This invention relates to impactors for the enrichment of aerosol particles and, in particular, to the use of virtual impactors for collecting the enriched aerosol particles.
2. Description of the Related Art
Impactors have been used for collecting aerosol particles for about 50 years now. The stream of gas containing the particles is accelerated towards a receptor plate by jet-forming nozzles in a generator plate. Due to their inertia the particles hit the receptor plate and are collected there whilst the gas deflects to the side. It is easy to realize that only heavy particles are collected whilst particles below a "cutoff threshold" size are conducted away by the stream of gas. A good overview is provided by the book entitled "Cascade Impactor, Sampling and Data Analysis", published by J. P. Lodge, Jr., and T. L. Chan, American Industrial Hygiene Association, Akron, USA (1986).
If the receptor plate, at the point where the particles would hit it, has a hole which will not take the gas stream from the jet, a "virtual impactor" is created. The particles enter the hole whilst the gas stream deflects to the side. Here too there is a "cutoff threshold" for the particles. If there is a chamber under the hole, particles with a size above the cutoff threshold can be collected in the chamber and be conducted away by a much smaller stream of gas. In this way it is possible to concentrate the particles into smaller gas flows. By applying this principle in a number of stages it is possible to achieve very high levels of enrichment where the particle concentration can reach several hundred times the initial concentration. A very good explanation of the principle and a theoretical calculation can be found in "Virtual Impactor: A Theoretical Study" by V. A. Marple and C. M. Chien, Environmental Science and Technology 14, 976 (1980).
For multistage impactors it is necessary to reduce the gas stream from stage to stage. To achieve consistent cutoff thresholds in the individual stages the nozzles and the collection conditions must remain the same from stage to stage. This means that there must be an extremely high number of nozzles in the first stage. The second stage requires far fewer nozzles, whilst the third stage requires only one in the limiting case. Good results with an enrichment of over 200 times can, for example, be achieved with 400 nozzles in the first stage, 20 in the second, and one nozzle in the third stage, in which the stream of air is reduced to about 1/400 in total.
The 400 nozzles in the generator plate of the first stage in the above example, and the corresponding receptor nozzles in the receptor plate, which must be adjusted in relation to one another with an exceptionally high degree of precision, are difficult to manufacture. Cleaning between operating phases is also difficult. Consequently, attempts have been made to replace the large number of circular individual nozzles by a single rectangular slit. A discussion of the properties can be found in a paper "Calibration of a High-Volume, Rectangular-Jet Virtual Impactor", T. J. Overcamp and D. E. Taylor, in "Aerosols In the Mining and Industrial Work Environments", Volume 3, published by V. A. Marple and B. Y. H. Liu, Ann Arbor Science, Butterworth Ltd, Sevenoaks, UK (1983).
A significant disadvantage of known slit-type nozzles is that the ends of the slit cause the impactor to operate with a high degree of instability. Also, in the vicinity of the end of the slit it is difficult to maintain the same cutoff limit which the rest of the slit produces. One proposal which has been made for overcoming the difficulty is to make the ends of the slit roughly heart-shaped (the slit is then roughly bone-shaped). However, this makes production and adjustment difficult. Furthermore, collection of particles in the impactor chamber causes difficulties with a long slit because the feed to the next stage becomes difficult and many particles tend to cling to the wall of the chamber. Having a number of short slits increases the problems at the ends of the slits.